The phrase “exhaust gas” covers the result of the combustion of a fuel mixture in an internal-combustion engine, stationary or installed in a motor vehicle, as well as any other type of combustion result, such as fumes from a turbine or an oven that circulate in a chimney.
In general, the ignition of a fuel such as a motor fuel creates, under high temperature and high oxygen content conditions, gases such as nitrogen monoxide (NO), nitrogen dioxide (NO2), more commonly referred to as NOx (nitrogen oxides).
These NOx are then discharged to the atmosphere and they present a danger, on the one hand, to people's health and, on the other hand, to the environment since they contribute to the formation of smog likely to attack the tropospheric ozone layer.
It is consequently necessary to provide means preventing discharge of these NOx into the atmosphere, for example by destroying them after they have formed.
To destroy these NOx, it is well known to conduct a chemical treatment operation such as a selective catalytic reduction, referred to as SCR (Selective Catalytic Reduction).
This operation consists in selectively reducing the NOx by means of a reducing agent on a dedicated catalyst. The reduction is referred to as selective because the reducing agent reduces the NOx and not the oxygen present in the mixture to be burned.
This reducing agent, in particular for use with an internal-combustion engine of a motor vehicle, is either ammonia (or a material that can decompose to ammonia), or a hydrocarbon, oxygenated or not, or a hydrocarbon mixture that can contain, partly or totally, one or more oxygenated hydrocarbons.
In the case of exhaust gas treatment with SCR using ammonia, the ammonia-based agent used is stored either in form of solid complexes, or in form of liquid precursors, such as urea in aqueous solution.
In the case of liquid precursors in form of urea, it is injected into the exhaust gas stream upstream from the catalyst. After this injection, the water contained in the urea is evaporated under the effect of the heat of the gas and the urea decomposes to ammonia and isocyanic acid.
For treatment with SCR using hydrocarbons, oxygenated or not, they are injected at the exhaust or they result from the combustion, in case of a late post-injection for example, and they react with the NOx contained in the exhaust gas. During this reaction, unstable complexes containing C, H, N and O form and they ideally decompose to carbon dioxide, water and dinitrogen.
Many other compounds however also form, such as hydrogen cyanide that can be present in an amount that is not insignificant, for example in the case of a treatment with SCR using ethanol.
As it is well known, devices allowing the injection of reducing agents to be managed are provided so as to ensure a sufficient injection for correctly reducing the NOx.
In this type of device, a gas detector notably sensitive to nitrogen oxides, more commonly known as NOx sonde, is arranged at the catalyst outlet and allows the amounts of gas discharged to be measured.
The output signal of this detector is sent to a computing unit intended for control and/or diagnosis of the reducing agent injection function and/or of the catalytic function. From this signal, the computing unit determines the amount of NOx contained in the exhaust gas at the catalyst outlet and this amount is then compared with a predetermined model. In case of a difference in relation to this model, the reducing agent injection setpoint is modified so as to reduce or even cancel this difference.
Although this device gives satisfactory results, it however involves considerable drawbacks.
Indeed, this device does not allow the presence of other compounds, such as isocyanic acid or hydrogen cyanide, to be detected.
The computing unit can thus not be informed of the presence of these compounds in the treated exhaust gas at the catalyst outlet and these compounds are discharged as they are to the atmosphere, where they can be harmful to living organisms.
The present invention aims to overcome the aforementioned drawbacks in a simple and inexpensive manner by using the devices already present on the exhaust line or the chimney.